Photosynthesis captures more carbon dioxide (CO2) from the atmosphere than any other process on Earth. Each year, land-based green plants capture about 403 gigatons (Gt) of CO2 (equivalent to 110 Gt C y−1) from the atmosphere into biomass. However, since biomass is not a stable form of carbon material, a substantial portion of the biomass decomposes in a relatively short time to CO2. As a result, increased biomass production (i.e., by increased tree growth) is of limited utility for carbon sequestration since the resulting biomass soon returns the absorbed CO2.
Unlike untreated biomass, carbonized biomass (i.e., charcoal or “biochar”) contains carbon in a highly stabilized state, i.e., as elemental carbon. The inertness of elemental carbon results in its very slow decomposition to CO2. Typically, at least several hundreds of years are necessary for the complete decomposition of biochar to CO2. As a result, there is great interest in producing biochar as a means for mitigating atmospheric CO2 production. There is particular interest in incorporating produced biochar into soil (i.e., as a soil amendment) where the biochar functions both as a CO2 sequestrant and as a soil amendment.
Biochar production and incorporation into soil has been practiced since ancient times. Of particular relevance is the recent discovery of biochar particles in soils formed by pre-Colombian indigenous agriculturalists in Amazonia, i.e., so-called “Terra Preta” soil. See, for example, B. Liang, et al., Soil Science Society of America Journal, vol. 70 (5), September-October (2006).
The capacity of carbon sequestration by application of biochar fertilizer is estimated to be quite significant. The amount of biochar materials that could be placed into soil could be as high as 10% by weight of the soil. Accordingly, in the first 30-cm layer of U.S. cropland soil alone, 40 Gt of carbon could be sequestered in the form of biochar particles. The worldwide capacity for storing biochar carbon in agricultural soils could exceed 400 Gt of carbon. A conversion as low as 8% of the annual terrestrial photosynthetic products (110 Gt C y−1) into stable biochar material would be sufficient to offset the entire amount (nearly 8 Gt C y−1) of CO2 emitted into the atmosphere annually from the use of fossil fuels.
Significant amounts of biochar are currently being produced as a byproduct in biomass-to-biofuel production processes. The most common biomass-to-biofuel production processes include low temperature and high temperature pyrolysis (i.e., gasification) processes. Pyrolysis operations generally entail combusting biomass in the substantial absence of oxygen. Biofuels commonly produced in low temperature pyrolysis operations include hydrogen, methane, and ethanol. Gasification processes are generally useful for producing syngas (i.e., H2 and CO).
An important property of biochar is its cation-exchanging ability. The cation-exchanging ability or lack thereof of a biochar is evident by the magnitude of its cation exchange capacity (CEC). It is known that biochar with, in particular, an increased cation exchange capacity generally possesses a greater nutrient retention capability. Biochars with greater cation exchange capacity generally possess a significant amount of hydrophilic oxygen-containing groups, such as phenolic and carboxylic groups, which impart the greater cation exchange ability (Liang et al., 2006, Ibid.).
The CEC is defined as the amount of exchangeable cations (e.g., K+, Na−, NH4+, Mg2+, Ca2+, Fe3+, Al3+, Ni2+, and Zn2+) bound to a sample of soil. CEC is often expressed as centimoles (cmol) or millimoles (mmol) of total or specific cations per kilogram (kg) of soil. A substantial lack of a cation-exchanging property is generally considered to be reflected in a CEC of less than 50 mmol/kg. A moderate CEC is typically considered to be within the range of above 50 and at or less than 250 mmol/kg. An atypically or exceptionally high CEC would be at least 250 mmol/kg.
Though biochar is generally useful for CO2 sequestration, the types of biochar found in ancient soils or produced as an industrial byproduct are highly variable in their physical and property characteristics, e.g., chemical composition, porosity, charge density, and CEC. One of the most common production processes of biochar is the practice since ancient times of burning biomass in open pits. Such uncontrolled processes generally produce significant quantities of oxide gases of combustion, such as CO2 and CO, generally in amounts significantly greater than 20 percent by weight of the carbon content of the biochar source. In addition, the resulting biochar is highly non-uniform in composition, e.g., substantially non-oxygenated portions particularly in the interior portions of the biochar pit and moderately oxygenated portions at the outer peripheral portions of the biochar pit. Furthermore, the uncontrolled process generally results in significant batch-to-batch variability. Moreover, by the uncontrolled process, the characteristics of the resulting biochar are generally unpredictable and not capable of being adjusted or optimized.
Though biochar materials possessing moderate cation exchange capacities are known, such biochar compositions are not typical, and moreover, are found sporadically and in unpredictable locations of the world. Therefore, there is a need in the art for a method for manufacturing oxygenated biochar compositions having at least a moderate cation exchange capacity so that such biochar compositions are more readily available. There is an additional need for such a method to produce oxygenated biochar compositions having an atypically high cation exchange capacity, and more preferably, a cation exchange capacity significantly higher than found in known soil deposits. Such biochar materials would have the advantage of more effectively retaining soil nutrients, and thus, functioning as superior fertilizing/soil amending materials while aiding in carbon sequestration.
There is an additional need for a method for producing oxygenated biochar wherein the biochar is reproducibly manufactured with low batch-to-batch variation in one or more characteristics of the biochar (e.g., cation exchange capacity, particle size, porosity, C:O ratio, and the like). There is a further need for a method for producing oxygenated biochar wherein the biochar is substantially uniform in one or more characteristics, such as oxygen-to-carbon ratio, CEC, and chemical composition. A further need exists wherein such a method can be appropriately adjusted, modified, or optimized in order to effect a corresponding modification, adjustment, or optimization in one or more biochar properties.